It has long been difficult to determine the quality of liquid fuels during transportation and storage. This has especially been true due to the presence of water or water-soluble components in non-aqueous liquid fuels, as the potential for water or water-soluble components to cause harm may be less dependent upon the amount of such components which are present than upon the conditions of transportation and storage.
Conventional methods for measuring water content within non-aqueous liquids usually fall into two categories, quantitative methods that require expensive equipment and labor, and simple methods that yield highly qualitative results. The quantitative approaches include analytical laboratory equipment and industry specific analyzers. While water-cut analyzers have been developed for the crude oil industry, these instruments are designed to measure water present in a separate phase from the crude oil. Simple methods, such as color changing indicator chemicals, may be highly portable and easy to use, but may not provide the information desired.
The conventional laboratory-based method for the measurement of water dissolved within non-aqueous liquids is Karl Fischer titration (see ASTM D 1744). While very accurate, the use of the Karl Fischer titration requires an expensive piece of equipment, the Karl Fischer titrator, and a trained technician as operator. In a facility or transport setting, standard analytical equipment is expensive, complicated, fragile, maintenance intensive, and requires trained technicians. Additionally, in a field setting, the instrument machinery may not be sufficiently compact, portable, and automated to permit practical use.
Indicator dye/colorimetric methods are known that use indicator materials that undergo changes in color when water or alcohol is present in a storage tank with petroleum fuels. U.S. Pat. No. 4,699,885 to Melpolder and Victor describes a paste that undergoes a change in color when exposed to a water phase. This invention is only capable of detecting a distinct aqueous phase and is not capable of detecting water dissolved within petroleum fuels. U.S. Pat. No. 4,604,345 to Felder and Panzer describes a paste that undergoes a change in color when exposed to a phase of alcohol or to petroleum fuels containing dissolved alcohol. Any water dissolved within the petroleum fuel must be removed by a drying agent for the paste to properly indicate the presence of alcohol. Neither invention is capable of producing reproducible quantitative measurements of water or alcohol concentrations in petroleum fuels. U.S. Pat. No. 5,229,295 to Travis describes colorimetric tests for the presence of water and ethanol, and prescribes a separate step for the volumetric determination of alcohol concentration. Due to the reagent handling and restocking requirements, none of these methods is well-suited for the automated measurement of the water or alcohol content of petroleum fuels. While easy to use by a non-technically trained operator, the information gained by these inventions is very limited.
Several patents have been granted to inventions that incorporate humidity sensors into their design. Modern relative humidity sensors are composed of an interdigitated gold terminal on an alumina substrate overcoated with a thermosetting hydrophilic polymer. This polymer is a polyelectrolyte blend exhibiting a change in ionic mobility as water of hydration is absorbed. The ionic mobility is a direct function of the water vapor pressure in the ambient environment as well as the ambient temperature. The operating principle was patented by Martin Pope (Pope M., U.S. Pat. No. 2,728,831, 1955) though recent iterations of his invention have proven to yield sensors of greater stability.
According to Henry's Law, the partial pressure of water vapor in equilibrium with a solution phase is directly proportional to the moisture content of the solution provided the solution is sufficiently dilute. The partial pressure of water vapor is equal to the relative humidity (RH) multiplied by the saturated vapor pressure of water at any given temperature. To a good approximation, the solvent relative humidity (SRH) above any hydrophobic liquid is equal to the relative humidity (RH) in air in the absence of the vapors of that liquid.
Therefore, it is possible to determine the concentration of dissolved water for hydrophobic liquids by the equation:C=(CS)*(SRH/100%)where C=water concentration in ppm
CS=saturated water concentration in ppm at a given temperature and pressure
SRH=solvent relative humidity as measured by the sensor
The measurement of SRH can either be made in the head space above the liquid or within the liquid itself since the chemical potential of the water is a function of either the concentration of water or the water vapor pressure above the solution.
U.S. Pat. No. 6,138,674 to Gull and Hunt describes a module which measures the humidity of a patient's expired respiratory gases for the purpose of compensating for these variables in the delivery of gaseous anesthetic. U.S. Pat. No. 6,039,696 to Bell describes an adapter for the measurement of the humidity of inspired and expired gases in a patient with an artificial airway. The adapter may act as a control device to assist the delivery of ventilating gases with physiological levels of moisture. The apparatus also includes a display means which receives signals from the humidity sensor, translates the signals, and displays the results as percent relative humidity and/or moisture content. U.S. Pat. No. 6,347,746 to Dage et al. describes the incorporation of a humidity sensor into a system which monitors the temperature and humidity of air in a vehicle for the purpose of detecting and preventing conditions which lead to the fogging of vehicle windows.
Patents have been issued to inventions which determine the water content of materials by measuring the electrical properties of the materials and relating these properties to water content. U.S. Pat. No. 4,786,873 to Sherman describes a method to determine the water content of hydrocarbon-containing porous earth formations by measuring the dielectric permittivity of the earth formations. U.S. Pat. No. 3,966,973 to Henry et al. describes a process by which the moisture content of food is obtained by measuring the impedance generated by the food passing through an alternating current field. U.S. Pat. No. 6,388,453 to Greer describes a swept-frequency shunt-mode dielectric sensor system is used to measure complex impedance parameters such as capacitance and/or dielectric loss of particulate materials in order to calculate density and water content. U.S. Pat. No. 6,664,796 to Wang describes a process by which the moisture content of a fuel containing exclusively ethanol, and concentration of ethanol in the fuel, is obtained by measuring the resistance of the fuel. Many patents have been issued that employ sensors of dielectric properties to measure the water content associated with hydrocarbon liquids, especially crude oil. U.S. Pat. No. 5,070,725 to Cox et al. describes a water-cut meter which measures the impedance associated with crude oil and water mixtures. The percentage of water may be determined in both water continuous and oil continuous samples. U.S. Pat. No. 5,260,667 to Garcia-Golding et al. describes a method for determining the water content of oil-in-water emulsions by measuring the real part of a sample's specific admittance and by making corrections for the sample temperature. None of the above methods for determining the water content of petroleum samples yield information specifically concerning the dissolved water content, but only the water in a separate phase from the petroleum or emulsified with it.
Methods of determining water content in oil streams also include microwave technologies. U.S. Pat. No. 4,862,060 to Scott et al. and U.S. Pat. No. 5,389,883 to Harper determine water content from the frequency changes between emitted and received microwave signals caused by the dielectric properties of oil and/or water samples. These methods do not determine the dissolved water content of the petroleum samples. Furthermore, microwave technologies are often expensive to implement.
However, despite the above, a need still exists for a single, field-capable, test method which can be used to determine the amount of water in or degree of water saturation of non-aqueous liquids (whose detailed composition can be changed without notice) and assess the potential for such water to cause problems during the storage and use of these liquids.